This invention relates to the preparation of particle dispersions in a media mill. More particularly, this invention relates to an improved media mill process wherein there is good fluidization of the media, predictable scale up behavior, efficient particle size reduction, low contamination and improvements to the performance of the final dispersion.
Media mills are being increasingly used for the milling of colorants in inks and coatings, such as automotive paints. Such mills can grind materials, but more typically they act to de-agglomerate particles and promote dispersion.
Conventional media mills used in commercial production usually comprise a chamber equipped with an agitator wherein milling is accomplished by fluidizing media with slurry. The slurry is usually fed continuously and there is usually a separator screen at the chamber exit to allow the passage of slurry and retention of media.
Smaller media allow more efficient milling and ultimately smaller particle size; however, as media gets smaller it becomes harder to separate, and pressure build up at the separator (due to hydraulic packing) becomes unacceptable. For most commercial applications, 0.35 mm media is considered the practical lower limit.
Better fluidization of the media provides faster, more efficient milling. The art of U.S. Pat. No. 5,022,592 (Zakheim et al.) is directed to improved fluidization.
For particularly demanding end use applications, such as the production of ink jet inks and automotive paints, the nature of the media is important. Friable media, such as glass and ceramics, can leave shards of fractured media in the dispersion that can clog narrow passages of ink jet printheads. Metallic media such as stainless steel and zirconium leave traces of metal in the dispersion that cause discoloration and, in some cases, electrical malfunction of the printhead. Zirconium contamination can also negatively affect appearance of automotive coatings by, for example, reducing transparency of aluminum- and mica-containing colors.
To avoid some of the problems of media contamination, U.S. Pat. No. 5,891,231 (Gnerlich et al.) teaches the use of plastic media preferably made of polystyrene and preferably spherical in shape.
The above-mentioned disclosures are incorporated by reference herein for all purposes as if fully set forth.
There is still need for improved media milling methods, especially for the production of colorant dispersions for high performance end use applications such as ink jet ink and exterior paints for vehicles such as automobiles and trucks.